The typical wastewater treatment plant includes a final clarifier in which the suspended solids from the prior treatment step are allowed to settle out. These solids generally have very low settling rates so it is important that the tank be designed to operate at its design average flow rate and also at the designed maximum flow rate. Proper operation includes maintenance of the headloss within the design ranges and uniform distribution of the liquid around the influent ports.
Clarifiers built before about 1958 were usually of the center feed type; since then, the demonstrable advantage of peripheral feed clarifiers, as disclosed in U.S. Pat. Nos. 2,961,099, 2,961,100 and 3,717,257 have made this type of clarifier preferred. In such clarifiers, the cross sectional flow area of the peripheral feed channel diminishes in the direction of flow, and is usually provided with a series of large ports which are not susceptible to clogging. Generally, all such ports are of the same size; the spacing between ports is selected to provide an equal distribution of the feed around the periphery of the basin.
The cross sectional flow area in relatively long influent channels must decrease in the direction of flow, and overflow launders must increase in cross section in their direction of flows so that neither the velocity of the flows nor the headloss becomes excessive. The change in cross sectional flow area of the channels is accomplished by varying their width along their length, or by sloping the channel floors.
The design of the clarifier must account for the wide range of flows, which include the minimum, average, maximum, and hydraulic peak. Normally a plant runs at the average (design) flow at 90% of the time for many years. Maximum or peak flow occurs due to wet weather or storm conditions. Also, when the sewage system is expanded the average or designed flow may be exceeded.
According to the current common practice, the orifices from the influent channels to the basin are designed for a given range of flow, and the headloss is a function of the orifice flow ratio, squared. This relationship is based on the following orifice equation: ##EQU1## Where: A.sub.O =Orifice Area
Q=Orifice Flow PA1 C=Coefficient of Discharge g=Acceleration due to Gravity (32.2 ft/sec.sup.2) PA1 H.sub.L =Orifice Headloss PA1 H.sub.LS =Set Orifice Headloss PA1 H.sub.LU =Unknown Orifice Headloss PA1 Q.sub.S =Orifice flow at set flow PA1 Q.sub.O =Orifice flow at other flow
When the headloss and area is set for one flow, the headloss at another flow can be determined by the equation: EQU H.sub.LS /Q.sup.2.sub.S =H.sub.Lu /Q.sup.2.sub.O
or EQU H.sub.Lu =H.sub.LS (Q.sup.2.sub.O /q.sup.2.sub.S)
Where:
For example, if the orifice flow is five units for the unknown peak to one unit for the set, and the set headloss is 0.2 feet, the unknown headloss is therefore: EQU 0.2(5.sup.2 /1.sup.2)=5.0 feet
In this example the hydraulic system would have to be sized for an additional 5 feet of headloss to accommodate the peak flow.
The headloss in a final clarifier directly affects capital and operating costs. A high headloss requires that the peripheral feed channel walls be high enough to contain the liquid at its maximum water level and also requires that the system upstream of the final clarifier be designed with sufficient head to ensure flow at the designed rate into the clarifier. This may require pumps and additional retaining wall heights which incresase operating (energy and maintenance) costs as well as increase capital costs.
The field has thus long been in need of a simple, inexpensive, and reliable system in a clarifier for handling the flow rate at maximum or peak flows without excessive headloss while allowing the clarifier to operate properly at the design or set flow rate.